PROJECT SUMMARY The retinoblastoma tumor suppressor (Rb) is a central gene that has been implicated in the formation of multiple solid tumors. It does so by both the regulation of cellular proliferation and driving cellular differentiation to establish cellular identity. Lack of Rb then leads to cancer, yet the mechanisms which underlie the pro- oncogenic consequences of Rb-loss are still not fully understood. A novel role for Rb in the repression of essential pluripotency genes has been identified and this repression of pluripotency serves to restrict cellular plasticity and tumor formation. One of the Rb-regulated pluripotency genes identified was Sox2 which can indeed be oncogenic in some cellular contexts. However, the mechanisms of Sox2 oncogenic activity in tumors formed by Rb-loss are currently unclear. This proposal is based upon the hypothesis that upregulation of Sox2 after Rb-loss is a crucial oncogenic driver in the Rb-loss initiated tumors, and is due in part to the ability of Sox2 to enhance cellular plasticity and upregulate progenitor-state pathways. To investigate this hypothesis, this project will aim to establish the role of Sox2 in cancer initiation upon loss of Rb. This will be achieved by the utilization of genetically engineered mouse models which will test if Sox2 is required for the initiation and growth of multiple Rb-loss initiated tumors. The role for Sox2 in the maintenance and propagation of these tumors will be studied by standard growth and transplantation assays using established mouse and human cell lines. Finally, this project will aim to elucidate the oncogenic mechanisms of Sox2 in cancer. Genomic-scale investigations into the gene network regulated by Sox2 will be examined by ChIP-seq of Sox2 in tumor cell lines in conjunction with RNA-seq, ChIP-seq of modified histones, and ATAC-seq to identify the means of regulation of the greater Sox2 network. Downstream targets will be validated by their overexpression or knockdown in tumor cell lines or GEMMs. This investigation will identify the extent that Rb-loss initiated tumors are dependent upon Sox2 and the mechanisms by which Sox2 participates in tumor development by the elucidation of the broader Sox2 network in cancer. This will serve to build a cohesive gene expression network to describe the oncogenic effects that follow Rb-loss and describe how these tumors are formed. Understanding this downstream Sox2 network may provide novel targets for the treatment of these tumors and will provide a groundwork for future investigations into the development of cancer therapeutics.